1. Field of the Invention
The present invention relates to an image forming system configuring a copying machine, a facsimile machine, a printing machine, a multifunction peripheral thereof, or the like, and more particularly, to a pseudo-halftoning device that performs dithering, which is one form of pseudo-halftoning, in an image forming system, and an image forming apparatus and an image forming system that include the pseudo-halftoning device.
2. Description of the Related Art
Image data to be input to an image forming apparatus as data for a tone image, such as a photograph, is generally multilevel data having 8 to 12 bits per pixel. As compared to this, the number of gray levels per pixel that can be expressed by image forming apparatuses (including electrophotography) for forming an image (so called a hardcopy) on paper is substantially considerably small. To accommodate such a problem, a hardcopy producing equipment increases resolution to 600 dots per inch (dpi), 1200 dpi, or the like and performs area coverage modulation to change image density with a plurality of pixels, thereby displaying a halftone image in a pseudo manner. Pseudo-halftoning is image processing to be performed in a process of converting input image data into a pseudo-halftone image.
Quantization of multilevel image data by dithering is described in detail in known patent documents and non-patent documents (particularly, see, Electrophotography, Vol. 24, No. 1, pages 51-59 (1985)). Hence, a pseudo-halftoning device employing dithering is briefly described below.
It is assumed that the pseudo-halftoning device employing dithering generates, from image data (having 8-bit (256 gray levels) data per color per pixel, for instance) before being subjected to pseudo-halftoning, image data (having 4-bit (16 gray levels) data per color per pixel, for instance) subjected to pseudo-halftoning by referring to a table called a dither matrix or a threshold matrix. A dither matrix for use in generating 4-bit data from 8-bit data as in this example includes 15 planes (the number of planes is smaller by one than 16, which is the number of gray levels after pseudo-halftoning). In each plane, one of 255 threshold values (the number of threshold values is smaller by one than 256, which is the number of gray levels before pseudo-halftoning) from 0 to 254 is assigned to each pixel.
A pseudo-halftoning method by dithering determines a value of each pixel in the image data after pseudo-halftoning in accordance with a comparative relationship between a value of pixel data of the image data before pseudo-halftoning and a threshold value assigned to each pixel in each plane of the dither matrix discussed above. Various types of dithering can be implemented by changing a way for assigning threshold values to each plane and a threshold value to each pixel in the dither matrix.
For instance, different types of halftoning, such as dot screen and line screen used for describing shapes of halftone dots, can be implemented by appropriately assigning threshold values to the dither matrix. Similarly, different sizes of periodic structure, such as screen ruling, and different types of halftoning, such as nonperiodic type, e.g., blue noise and green noise, can be implemented by appropriately assigning threshold values to the dither matrix.
An image forming apparatus typically includes, in addition to such a pseudo-halftoning device as discussed above, a tone correction device. A major function of the tone correction device is to perform tone correction on an input image while taking output characteristics of the image forming apparatus into consideration so that the input image is output with intended gradation characteristics. The tone correction is typically performed by converting input image data into output image data having the same number of gray levels (generally, the number of gray levels of an input image is 8 bits; accordingly, the tone correction is typically conversion from 8 bits into 8 bits). Such a tone correction places a limit that a tone-corrected image can be reproduced only at the number of levels equal to or fewer than 8 bits. This can cause so called gray-level missing to occur. This gray-level missing can produce a segment where a large tone jump occurs, or the like, which is a factor of degrading image quality.
As one of measures for preventing or reducing the gray-level missing, methods of incorporating tone conversion in a dither matrix have been proposed. A technique corresponding to such a method is disclosed in, for instance, Japanese Patent Application Laid-open No. 2001-61064.
The reason why incorporating tone conversion in a dither matrix can reduce gray-level missing is described below. A dither matrix has a degree of freedom in the number of gray levels equal to the number of positions, to each of which a threshold value can be assigned. The number of positions, to each of which a threshold value can be assigned, is equal to a product of the number of planes in the dither matrix and the number of pixels in each plane. It is relatively easy to set the number of pixels in each plane to a large value by using a super matrix (a dither matrix subsuming a plurality of basic dither periods). Accordingly, by using a dither matrix having a large number of pixels, a dither matrix having a degree of freedom in the number of gray levels greater than the number of gray levels of an input image can be generated easily and used.
A dither matrix incorporating tone conversion can be obtained by arranging 255 threshold values from 0 to 254 in a dither matrix having a large degree of freedom in the number of gray levels in such a density distribution as to acquire intended gradation characteristics. For instance, a dither method that incorporates intended tone conversion can be implemented by setting, for an area where a change in the gray levels in an output image needs to be small, the number of threshold values assigned to the area relatively small, whereas setting, for an area where a change in the gray levels in an output image needs to be large, the number of threshold values assigned to the area relatively large.
The method of incorporating tone conversion in a dither matrix as discussed above is equivalent to extraction of necessary gray levels (256 gray levels) from gray levels that can be realized by using a threshold matrix having a large degree of freedom in the number of gray levels. Put another way, the method is equivalent to tone conversion with a relatively large number of gray levels; hence, gray-level missing can be reduced.
Meanwhile, Japanese Patent Application Laid-open No. 2001-61064 discloses an image processing apparatus which performs multilevel dithering by using a reference threshold array and in which gamma conversion for converting image data into image data having target output characteristics is incorporated in threshold values of each dither threshold plane used in the multilevel dithering. It is discussed in Japanese Patent Application Laid-open No. 2001-61064 that with conventional dither method, interference between different colors produces a moire pattern, such as a rosette pattern, or visually noticeable unintended texture at a specific gray level; hence, there are many problems to be solved to obtain optimum output characteristics across all gray levels. It is discussed that the image processing apparatus configured as discussed above serves as a solution to the conventional problems and can improve tone reproduction.
In Japanese Patent Application Laid-open No. 2006-49992, an image forming apparatus including an edge detection unit that detects edges in an input image by forming an image matrix and by using an edge-detection filter and a solid-image-portion detection unit that detects a solid-image portion by calculating a range of variation in neighboring pixel data and comparing the range of variation with a preset threshold value. The image forming apparatus selectively performs pseudo-halftoning appropriate for a target image portion based on a result of edge detection performed by the edge detection unit and a result of solid-image portion detection performed by the solid-image-portion detection unit. It is discussed in Japanese Patent Application Laid-open No. 2006-49992 that although a conventional technique enables processing to be performed selectively between a character portion and a non-character portion, the technique has been disadvantageous in not allowing different processing to be performed for a solid-image portion that can be detected. In contrast, it is described that the image processing apparatus configured as discussed above is capable of pseudo-halftoning in a manner optimum in both sharpness of characters and smoothness of solid-image portions for each image portion.
Japanese Patent No. 3823703 discloses an image processing apparatus including a sampling unit that performs sampling on each of color components of input multilevel image data corresponding to a threshold matrix not having a screen angle, an interpolating unit that generates image data sampled corresponding to a threshold matrix having a screen angle from image data of color components other than one color component among sampled image data, and a pseudo-halftoning unit that performs area coverage modulation on the image data of the one color component sampled by the sampling unit by using a threshold matrix not having a screen angle and performs area coverage modulation on the image data of the other color components generated by the interpolating unit by using a threshold matrix having a screen angle.
It is discussed in Japanese Patent No. 3823703 that an image processing apparatus having a conventional configuration has been disadvantageous in that, to perform halftoning by using a multiple types of threshold matrices that differ from one another in screen angle, it has been necessary for the image processing apparatus to include, individually for each of the screen angles, units for removing high-frequency components, which results in an increase in size and cost of the image processing apparatus that performs area coverage modulation. In contrast, the image processing apparatus configured as discussed above is capable of area coverage modulation by using threshold matrices having screen angles without storing image data about two-dimensional positions corresponding to the threshold matrices having screen angles in advance, so that it becomes easy to realize size reduction, cost reduction, and the like of the apparatus as well as speedup in processing, and furthermore versatility of area coverage modulation can be sufficiently ensured by facilitating a process for changing a screen angle.
An image forming apparatus is required to output an image in the same colors (in the same lightness or in the same density when the image is a monochrome image) as those of input image data. Accordingly, an image forming apparatus is designed to maintain colors of output images constant. However, to be more exact, colors of output images for a single input image data are not completely identical with each other depending on an image forming position, and the like, and are reproduced in different colors. Such a phenomenon, in which in spite of the fact that images are desirably reproduced in the same colors, the images are undesirably reproduced in different colors for some reason, is referred to as color fluctuation below.
The color fluctuation can be roughly classified into two types.
A first type is color fluctuation with certain regularity. This type of color fluctuation with regularity can be caused by various causes. For instance, variation in light intensity that depends on a position in the main-scanning direction in optical writing performed in laser raster scan (exposure method typically used in an electrophotographic image forming apparatus) causes the first type of color fluctuation. Other examples of the cause for the color fluctuation with regularity include variation in thickness of a photosensitive layer on a photosensitive drum and a decentering of the photosensitive drum (decentering of the drum causes development gap or the like to change regularly). Attempts for reducing this type of color fluctuation with regularity have already been made because it is relatively easy to determine a cause therefor and to take a countermeasure. However, taking a sufficient countermeasure against the color fluctuation leads to addition of a mechanism, which results in an increase in cost. Furthermore, in recent years, further reduction in color fluctuation has been desired because requests for output images in invariant colors have increased than in the past. Thus, a technique that allows reduction in color fluctuation less expensively is eagerly desired.
The other type of the color fluctuation is color fluctuation without regularity. This type of color fluctuation without regularity is caused by various causes as well; however, it is difficult in many cases to determine a cause therefor because of the irregularity. Examples of this type of color fluctuation include color fluctuation caused by environmental changes, such as ambient temperatures and humidity, and color fluctuation caused by usage patterns in the past (caused by a change in characteristics of a part over long term use or the like).
Disclosed in Japanese Patent Application Laid-open No. 2001-61064 is the technique of performing gamma conversion (corresponding to tone conversion in the present invention) by incorporating it in threshold values in each of dither threshold planes for use in multilevel dithering. However, this technique aims to obtain optimum output characteristics across all gray levels rather than solving the color fluctuation problem. Hence, although gamma conversion is incorporated in the dither threshold planes, the threshold planes incorporating the gamma conversion are applied to every image segment, so that this technique fails to solve a problem, resulting from the color fluctuation, that gradation characteristics vary on a per-image-segment (per-image-position) basis.
Disclosed in Japanese Patent Application Laid-open No. 2006-49992 is the technique of applying different dither matrices on an image-segment-by-image-segment basis. However, this technique aims to attain both sharpness of characters and smoothness of solid-image portions. In other words, only two types of dither matrices for character image portions and solid-image portions are provided; therefore, a technique of applying different dither matrices individually to each of three or more types of image segments is not described. Hence, this technique, fails to solve the problem, resulting from the color fluctuation, that gradation characteristics vary on a per-image-segment (per-image-position) basis.
Disclosed in Japanese Patent No. 3823703 is the technique of including the interpolating unit that interpolates image data. However, a technique for solving the problem, resulting from the color fluctuation, that gradation characteristics vary on a per-image-segment (per-image-position) basis is not referred to in Japanese Patent No. 3823703. Hence, the technique disclosed in Japanese Patent No. 3823703 fails to solve the problem, resulting from the color fluctuation, that gradation characteristics vary on a per-image-segment (per-image-position) basis.